COLD HARDINESS AND ITS RELATIONSHIP TO OVERWINTERING OF THE RUSSIAN WHEAT APHID (HOMOPTERA, APHIDIDAE) IN SOUTHERN ALBERTA

Russian wheat aphid, Diuraphis noxia (Mordvilko), is a new pest of winter wheat, Triticum aestivum L., in western Canada. Studies were conducted to compare the cold hardiness of Russian wheat aphid, as expressed by the supercooling point, with overwintering mortality in the field. Mean supercooling points ranged from -26.8-degrees-C for first instars to -24.9-degrees-C for adults. These differences were not reflected in the winter survival of each stage in the field. Generally, aphid numbers declined steadily at surface temperatures between 0 and -10-degrees-C. In 1988-1989, temperatures at 1 cm above the soil surface dropped lower than -25-degrees-C for 4 d on one occasion, resulting in 100% mortality of Russian wheat aphid. In 1989-1990, when temperatures did not drop below -25-degrees-C, a low number of Russian wheat aphids were able to overwinter successfully. For the Russian wheat aphid, the supercooling point and the ability to survive cold temperatures are not closely related. Although extremely cold temperatures result in extinction of exposed populations, most winter mortality occurs at temperatures above the supercooling point.